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The ferrofluid infused into this FLIPS collects in the center due to a magnet placed below the material. The level of ferrofluid gets depleted in other areas (orange areas), partially exposing a rough surface that can trap a water droplet until the magnetic field is removed.

With the help of a magnetic fluid, researchers have designed surfaces with tunable friction, allowing them to control the movement of liquid droplets and other particles. Such surfaces could enable microfluidic devices controlled by magnetic fields (Nature 2018, DOI: 10.1038/s41586-018-0250-8).

Credit: Wendong Wang/Nature

A magnet placed under this FLIPS surface has caused diluted ferrofluid to collect in a small area (dark brown), allowing researchers to trap droplets of a dye solution and assemble them into clusters. Researchers control the number of droplets per cluster by changing the distance between the magnet and surface.

Harvard University’s Joanna Aizenberg and coworkers created the surfaces by infusing a ferrofluid—a suspension of magnetic particles in a variety of liquids—into a microstructured, porous epoxy surface. The team previously had made slippery liquid-infused porous surfaces, or SLIPS, by adding lubricants to similar porous solids. What’s different about these new surfaces—called ferrofluid-containing liquid-infused porous surfaces (FLIPS)—is that they allow researchers to control the distribution of the fluid within the structure with a magnetic field.

A FLIPS surface is flat and slippery until researchers place a magnet nearby. The magnetic field pulls the ferrofluid and makes it form a variety of different configurations, depending on the underlying pattern on the microstructured surface and other factors.

As a result, the magnetic field depletes the ferrofluid in one area of the surface, exposing the roughness of the epoxy solid underneath and effectively “turning on” the surface’s friction. The team used this tunable friction to temporarily pin a water droplet in place; when the researchers released the magnetic field, the droplet slid away.

The team demonstrated several other functions for the FLIPS, ranging from those on a very small scale to some on a larger scale. For example, they transported nonmagnetic colloidal particles, pumped a dyed ethanol solution, and removed a biofilm of green algae that had accumulated on a FLIPS surface.

The study transforms a simple concept into “a versatile technique for manipulating various objects and phenomena,” says Liming Dai of Case Western Reserve University, who designs materials with functional structures and smart features.

Aizenberg says her team continues to work to better understand the underlying physical phenomena of how these fluids interact with microscructures and develop dynamic materials for various applications. In particular, the group is interested in exploring whether FLIPS surfaces could enable self-cleaning pipes that prevent the buildup of algae.